The present application claims priority under 35 U.S.C. xc2xa7 119 of German patent application No. 199 35 138.4, filed on Jul. 27, 1999, the disclosure of which is expressly incorporated by reference herein in its entirety.
1. Field of the Invention
The invention relates to a drying section of a machine for producing a material web, e.g., a paper or cardboard web.
2. Discussion of Background Information
In conventional multi-cylinder drying sections, the paper web to be dried is guided over a number of steam-heated cylinders or over a configuration involving a number of steam-heated cylinders and screen suction rolls. Particularly at the outset of drying, when the paper web still lacks adequate strength, there are frequently problems with respect to web guidance, which can be attributed in particular to the fact that the still-moist web continues to stick to the smooth contact areas, areas that are urgently necessary for adequate heat transfer. These problems can be reduced by coating the cylinders. Relatively high tension forces are therefore necessary, though this frequently leads to web tears and overstretching of the web edges. Therefore, as a rule, therefore, it is necessary for the drying to proceed more slowly, i.e., with, for example, a lower steam pressure at the beginning, and this means that the required drying section becomes longer overall. The problems discussed above arise to a greater degree at high web speeds.
The present invention relates to a drying section of the type described at the outset which, at high machine speeds, provides as optimal a drying rate as possible at the beginning of drying, as well as a secure guidance of the web.
Accordingly, the material web, starting at the last press nip of a press section, is conducted in a closed path over at least one straight or slightly curved section, to a larger support roll with an assigned impingement dryer, which is, arranged to subject the material web to an impingement of hot air or hot steam, as it is guided over the support roll. The straight or slightly curved section is provided in the form of an impingement drying section and/or a transfer foil section.
With this design, it is no longer necessary to guide the still relatively moist material web over smooth contact surfaces in order to achieve adequate heat transfer. Because smooth contact areas are avoided at the beginning of drying, the danger of tears in the web and overstretching of the web edges is practically eliminated. Higher drying rates, in particular, are now possible, and this in turn shortens the overall length of the drying section. In principle, impingement drying can also take place in any given transfer foil section. This kind of transfer foil section can be provided in the form of a simple transfer section.
In a preferred embodiment of the drying section according to the invention, the material web is supported at least in the area between the last press nip of the press section, or the removal from the press felt or press belt belonging to the last press nip, and the support roll, by an open, i.e., not smooth, support surface which can be formed, e.g., by a continuous belt or screen. Here, the material web is preferably also supported in the area of the impingement dryer assigned to the support roll by an open, i.e., not smooth, support area, which is being formed either by the surface, or coating or cover, of the support roll or by the surface of a continuous belt or screen guided over the support roll.
It is advantageous for the support roll to have an outer diameter ranging from about 2.5 to about 8 m, and preferably ranging from about 3 to about 5 m.
In a useful practical embodiment, the support roll assigned to the impingement dryer is provided with suction, and a continuous screen is guided over the support roll in order to hold the material web in place. This can be done directly by a suction box located within the device or through openings provided in the jacket surface by an external suction box. When the diameter increases, not only does the circumference of the support roll increase, but the possible angle of wrapping covered by the impingement dryer also increases, and thus the drying area. The impingement dryer can lie above or below the cylinder plane and can be moved for the purpose of transfer and/or cleaning, especially in parts. When the support roll is provided with suction, the roll and the corresponding suction box can contain narrow transfer zones for the purpose of strip transfer.
If impingement flow drying occurs not only in the area of at least one larger support roll but also in that of the straight or slightly curved section, the major part of the impingement drying is ideally taken over by the impingement dryer assigned to the larger support roll.
In principle, a plurality of impingement dryers, each assigned to a given support roll, can also be provided.
In a useful practical embodiment, the material web, following at least one impingement dryer assigned to a given support roll, is conducted to a drying group with at least one drying cylinder. Here, the material web can be fully dried in a single-row or double-row drying group.
On reaching or leaving the first drying cylinder or upon being withdrawn from a smooth heated surface, the material web ideally will already have a dryness content that is greater than about 55 to 65% wt. It is thus possible to avoid pulling the web from a smooth heated area during the time the web has a dryness content of less than about 55 to 65% wt.
In a preferred practical embodiment of the drying section according to the invention, the material web can be moved in the web travel direction and into the cellar, at a point in front of the support roll following the straight or slightly curved impingement drying or transfer-foil section with the assigned impingement dryer, i.e., it can be moved out of the machine (e.g., into the pulper). Thus, refuse removal is provided after the press section.
Here, the material web can be moved into the cellar, e.g., at a point immediately beyond an initial (as viewed in the web travel direction) straight or slightly curved impingement drying section or transfer-foil section.
Alternatively or additionally, the material web can also be moved into the cellar in the vicinity of a pickup roll by which the material web can be removed from a press felt or press belt guided through the last press nip of the press section when the material web is not being conducted into the drying section by the preceding pickup roll.
In certain cases, it is also advantageous if the material web can be moved into the cellar in the area of an additional press roll which lies opposite a deflection roll which is provided for a press felt guided through the last press nip and which is located behind the last press nip of the press section.
As an alternative, the web can also be moved into the cellar in the area of a smooth removal roll which is encircled by the last press felt or belt in such a way that the web runs in conjunction with the roll. A removal scraper is preferably installed on this roll.
It is useful to provide at least one straight or slightly curved impingement drying section or transfer-foil section in which the material web is guided between two screens. In a corresponding straight or slightly curved impingement dryer section or transfer-foil section the material web can be exposed to impingement while passing through the upper screen and/or the lower screen. The screen provided on the blow side should be resistant to high temperatures and be as permeable as possible to the impingement. The material web and the two screens can be supported, e.g., by support rolls, which are only encircled to the extent necessary for paper damage not to arise due to sheering, e.g., an angle of wrapping in a range from about 2xc2x0 to about 10xc2x0 can be provided, particularly from about 3xc2x0 to about 7xc2x0. When there is a plurality of impingement dryers, which can be provided in the form of, e.g., impingement boxes, the blow side can be alternated.
In principle, at least one straight or slightly curved impingement dryer section or transfer foil section can be provided in which the material web is held onto a screen by transfer foils. The second or covering screen can be eliminated. If there is impingement drying on this kind of section, blowing can occur directly from the given impingement dryer or impingement box onto the material web. The given transfer foil section can also be provided as a pure transfer section, however, without impingement drying.
The impingement drying or transfer foil sections can run horizontally or at an incline.
A web cut between the press section and the first larger support roll with its assigned impingement dryer can be performed, e.g., following the, first double-screen section, from a screen into the pulper by a movable roll and supported by water or air jets positioned behind the screen; or by another pickup roll at an additional removal point at a second lower felt; or by another press nip, namely at an opened pickup point into the drying section.
In certain cases, it is useful for at least two impingement dryers positioned on the same side of the material web to be provided. Here, at least one such impingement dryer can also be assigned to a straight or slightly curved impingement drying section or transfer foil section.
In certain cases, it is also useful for at least two impingement dryers to be provided on different sides of the material web. Here, too, at least one impingement dryer can be assigned to a straight or slightly curved impingement drying section or transfer foil section.
In a useful embodiment, at least one straight or slightly curved impingement drying or transfer foil section is provided between two impingement dryers, each of which is assigned a support roll.
In certain cases, it can be useful for at least one impingement dryer to be provided in an area other than the direct starting area of the drying section, i.e., following one or several drying cylinders belonging to the drying section.
Particularly in the case of several large support rolls with assigned impingement dryers, drying can be performed in an alternating manner, or it can always be performed from the same side and a transfer between the larger support rolls can be made through a straight or slightly curved impingement drying section or transfer foil section.
It is advantageous for at least one impingement dryer to be provided with at least one dryer hood.
In a preferred practical embodiment of the drying section according to the invention, the material web is delivered from a screen belonging to a straight or slightly curved impingement drying section or transfer foil section to a screen conducted over the support roll, e.g., the delivery area can be provided with suction by a suction box that is provided in front of or upon the support roll.
It can be useful for air that is entrained by the screen to be stripped in front of the intake into the nip, which is possible, e.g., with a flexible blade, or the air can be provided with suction from the box in a preliminary zone. In addition, the removal can be supported by blow jets that either blow on the web through the screen, e.g., in the case of blow jets integrated into the small suction roll or into the intermediate space thereafter, or by blow jets that operate from the hood rim and blow the web to the large support roll, which is provided with suction. One part of the small suction roll can take the form of an overpressure zone; or an overpressure box can be installed in the intermediate space, in order to push the web back away from the preceding screen.
In an alternative and useful embodiment, a screen assigned to a straight or slightly curved impingement drying section or transfer foil section is simultaneously guided over the support roll while, in the area of a guide or blow roll, the material web is brought up onto the support roll.
Thus the material web can be guided, for example, directly onto the large support roll, which is preferably provided with suction, via a web-guiding roll or via a blow roll which assures that the web is successfully detached. In front of the web approach point, the guide roll can be outfitted with jets which produce an air pillow and thus assure optimal removal of the web. The jets can be integrated into a scraper. In the case of a blow roll, the latter can be covered with a screen stocking.
When the hood is positioned below, the material web can, after the larger support roll with the screen is reached, be guided directly onto an unheated web-guiding roll or a drying cylinder. The scraper should be combined with a deflection sheet or with the hood in such a way that, even when the hood is open, paper does not fall from the scraper into the hood or result in blockages. The web-guiding roll can also take the form of, e.g., a blow roll, possibly with a screen stocking, which results in easy removal of the web.
When the hood is positioned above, the removal of the material web from the screen of the larger support roll can be performed with a suction roll, namely in such a way that, when there is a tear, paper can fall between the screens into the cellar.
The transfer between two support rolls that are provided with suction and positioned in succession can occur in an analogous fashion.
In a given instance of impingement drying, the following impingement conditions can apply, e.g., blow temperature about 200 to 400xc2x0 C., air speed about 70 to about 140 m/s, preferably about 90 to 120 m/s, distance from the impingement dryer or the hood about 10 to about 70 mm, preferably about 30 to about 50 mm. The air speed can be varied according to zones for the sake of cross-profiling, which also applies to the air temperature and moisture. In addition, the conditions in the different boxes and hoods, and in the longitudinal areas, can be differently adjusted, e.g., in order to permit the temperature in the dual-screen area to be adjusted to the load capacity of the screen or to permit a lower moisture to be selected, so as to avoid condensation when the material web is insufficiently warm. For the sake of effective energy usage, the blown air can be guided in cascade fashion. The impingement hoods on one or a plurality of large support rolls perform the major part of the impingement drying. Instead of hot air, superheated steam can also be used as a drying medium. In principle, the use of both media is possible.
Depending on the diameter of the larger support roll, it may be advantageous if some of the subsequent drying cylinders are positioned at a lower level. To further increase the output in the main vaporization process, further impingement dryers or impingement hoods can be inserted in the area of the cylinder drying section, specifically on larger suction rolls above or below the cylinder plane. The individual larger support rolls can have differing diameters. If the refuse removal following the press is not performed with a second pickup roll on the last press felt, but rather on a straight or slightly curved impingement drying section or transfer foil section, then any embodiment variant can be combined with the material web being removed from an upper press felt. Additional IR emitters for rapid heating can be installed in the straight area where the impingement hood begins.
The present invention is directed to a drying section of a machine for producing a material web. The drying section includes a support roll with an associated impingement dryer, and at least one straight or curved section forming at least a part of a closed path between a last nip of a press section adjacent the drying section and the support roll. The support roll and the impingement dryer are arranged to guide the material web over the support roll, and, thereby, to subject the material web to at least one of hot air and hot steam impingement. The at least one straight or slightly curved section is structured and arranged to form at least one of an impingement drying section and a transfer foil section.
According to a feature of the instant invention, the material web can include one of a paper and cardboard web.
In accordance with another feature of the invention, one of a continuous belt or screen can be arranged to form an open support area, at least in an area between the last press nip and the support roll. Further, the open support area is not smooth.
In accordance with still another feature of the present invention, another open support area can be formed in an area of the impingement dryer. The another open support area may be formed one of by the surface, covering or coating of the support roll and by a surface of a one of a continuous belt and screen guided over the support roll.
The support roll can have an outer diameter in a range between about 2.5 to about 8 m, preferably, the support roll has an outer diameter in a range between about 3 to about 5 m.
According to a further feature of the invention, a plurality of impingement dryers can each be associated with a support roll.
Further, a dryer section can include at least one drying cylinder which is arranged downstream from the support roll, relative to a web run direction.
The material web, upon one of leaving the first drying cylinder and being drawn from a smooth heated surface, can have a dryness content that is greater than about 55 to about 65% wt.
In accordance with another feature of the invention, a cellar may be positioned in front of the support roll, relative to a web run direction. Further, the cellar may be arranged to follow the at least one straight or slightly curved section, relative to a web run direction. Still further, an additional pick-up roll can be arranged to transport the material web into the cellar. The material web may be removed from one of a press felt guided through the last press nip and a press belt. The press belt is one of an open and closed belt. Further still, an additional press roll and a deflection roll can be arranged opposite each other. The deflection roll may be located within a press felt loop which passes through the last press nip, and the additional press roll may be arranged to transport the material web from the press felt to the cellar. A smooth removal roll can be encircled by one of a press felt and a press belt may be guided through the last press nip and that is arranged to transport the material web into the cellar.
According to a still further feature of the instant invention, the at least one straight or slightly curved section may include two screens arranged such that the material web is guided through the two screens.
Moreover, the at least one straight or slightly curved section can include a screen and transfer foils, which are arranged to hold the material web onto the screen.
In accordance with still another feature of the present invention, the support roll may include a suction roll and a narrow strip transfer zone. The suction roll can be coupled to one of an internal suction box and an external suction box.
According to another feature of the instant invention, at least two impingement dryers can be positioned on a same side of the material web.
In accordance with still another feature of the present invention, at least two impingement dryers can be positioned on different sides of the material web.
The at least one straight or slightly curved section can be arranged between two impingement dryers which are each assigned to a support roll.
Further, the at least one impingement dryer may be arranged in an area which is not at a direct starting area of the drying section. The at least one impingement dryer can be arranged only after at least one a plurality of drying cylinders.
According to a further feature of the invention, the at least one impingement dryer may include at least one dryer hood. The at least one hood can be swivelable. The swivelable hood can be coupled to a tear control system.
In accordance with a still further feature of the present invention, the at least one straight or slightly curved section can include a screen and the material web is transferred from the screen to a support roll screen guided over the support roll. A transfer area includes a suction box arranged one of in front of and on the support roll.
In accordance with yet another feature of the present invention, the at least one straight or slightly curved section can include a screen which is simultaneously guided over the support roll. The material web can be guided onto the support roll in an area of one of a guide and blow roll.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.